Field of the Invention
The present invention relates to locating incipient faults that generate partial discharges.
More in particular, the present invention relates to a method for locating incipient faults that generate partial discharges in an electric power transmission system, in particular a medium/high voltage electric power transmission system, as well as to an apparatus for carrying out the method.
Background of the Related Art
In the present description and claims the terms:                “Medium Voltage” or MV is used to indicate voltages in a range from 1 to 35 kV;        “High Voltage” or HV is used to indicate voltages higher than 35 kV;        “incipient fault” is used to indicate a defect within an electric power transmission system, in particular within a cable, which will not cause immediate failure but which may lead to possible failure;        “locating” is used to indicate identifying a certain portion of power transmission system containing the incipient fault, the length of the portion representing the accuracy of the locating;        “partial discharge”, sometimes shortened to PD hereinafter, is used to indicate a localized electrical discharge that partially bridges an insulation between conductors, and that can or cannot occur adjacent to a conductor.        
Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having duration of much less than 1 μs. As insulation it is meant, for example, the insulating layer surrounding a conductor.
For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about”. Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.
In the present description and claims, the terms “conductive”, “insulated”, “connected” and other terms that might also have a thermal or mechanical meaning are used in the electrical meaning, unless otherwise specified.
Electric power from a power generating plant is typically transmitted to a user, such as a city, a factory or other entity, by means of medium/high voltage electric power transmission systems that may be aerial (overhead), terrestrial or submarine.
An electric power transmission system, or network, can comprise electric conductors (that may be aerial (overhead) conductors, insulated terrestrial conductors or insulated submarine conductors), junctions, terminations, insulators for aerial power systems, Gas Insulated Lines (GILs) and/or Gas Insulated Switches (GISes). As far as the aerial conductors are concerned, the invention relates to insulated aerial cables.
A partial discharge usually begins within voids, cracks, inclusions or other defects within a solid dielectric, at conductor-dielectric interfaces within solid or liquid dielectrics, or in bubbles within liquid dielectrics. Partial discharges can also occur along the boundary between different insulating materials.
As mentioned, partial discharges usually do not cause the immediate failure of the electrical system, rather its progressive deterioration, ultimately leading to electrical breakdown.
In electric power transmission systems, particularly medium and high voltage transmission lines, relatively small damage or defects to components, especially to cable portions, for example conductors, insulators or sheathing, can lead to appreciable current losses. Moreover, such damage tends to increase or propagate and may become the cause of short-circuits which lead to a switching-off of the relevant network portion, and under certain circumstances may even have the consequence of still greater damage to the network, causing correspondingly greater expenditure in repair, service interruption, etc.
It is therefore important to detect and monitor partial discharges in an electric power transmission system in order to ensure reliable, long-term operation of the system, to predict possible failures that might lead to the interruption of the power supply service, and to schedule suitable in situ checks and/or changes of the component that is generating the partial discharge activity, before its failure.
Due to the wide extension of an electric network and/or to the often difficult accessibility thereof, especially when it is underground or overhead, it is important to locate as precisely as possible the site of the incipient fault that generates the partial discharge.
Techniques to detect and locate partial discharges in an electric power transmission system are known in the art.
Some techniques use Time-Domain Reflectometry (TDR), a measurement technique used to determine the characteristics of electrical lines by observing reflected waveforms. TDR is based on the injection and propagation of a step or impulse of energy into a system, and the subsequent observation of the energy reflected by the system. The accuracy of this technique in locating an incipient fault generating partial discharges in general depends on several factors, including the length of the link, the accuracy in the velocity of propagation value that is chosen for the calculation, and the distortion of the pulses at the measurement point due to dispersion.
The accuracy may thus not be fully satisfactory, especially in long and/or complex power networks. Indeed, partial discharges need short energy impulses to be detected, but long cable portions need long energy impulses to be run through.
TDR provides the distance of the defect from the measurement point, which is usually an end of a cable. It is however not easy at all to know where such a point will actually be because of the three-dimensional, often irregular arrangement of the cable.
Among techniques using TDR, for example, U.S. Pat. No. 6,853,196 relates to a method for determining the location of a defect site in a cable which comprises injecting into the drive end of the cable a short-duration incident pulse having sufficient voltage to cause an electrical breakdown at the defect site. The electrical breakdown, in turn, generates pulses which propagate away from the defect site in both directions along the cable. At least one diagnostic sensor can detect the incident pulse and the breakdown-induced reflected pulse at the drive end of the cable. The location of the defect site is estimated from the time delay between the arrival times of the pulses and the propagation velocity of the pulses in the cable.
Other methods provide for allocating detecting sensors operatively connected to various points along the entire electric network. Taking into account the extension of an electric network, such methods result to be rather expensive.
For example, WO 2009/013639 relates to a method for detecting, identifying and locating partial discharges occurring in a discharge site along an electric apparatus. The method comprises a preliminary step of identifying a plurality of detection stations along the entire apparatus to be evaluated, whereat the sensor can be coupled for the detection of the electrical signals. The partial discharges are located at the station where the signals have maximum values of an amplitude parameter and of a shape parameter correlated to the frequency content of the signals.
WO 2009/150627 discloses a portable partial discharge detection device for detecting and measuring partial discharges in electrical components and apparatus, which delivers signals having a form much resembling that of the radiated pulse, and which can also detect and deliver a sync signal which is obtained by picking up the supply voltage of the discharge generating source object. The device comprises a wide-band antenna suitable for acting as an electric field sensor and comprising a first planar conductor cooperating with a second conductor whose profile converges towards the first planar conductor at one point or along a line, wherein said second conductor is smaller by about two orders of magnitude than the field wavelength to be detected, so that the wide-band antenna is non-resonant in a band from about 0.1 MHz to about 100 MHz.
U.S. Pat. No. 4,967,158 relates to a portable detector device for detecting partial electrical discharge in live voltage distribution cables and/or equipment. The device comprises a probe secured to an insulated portable handle for manual displacement of the probe. A detector device is connected to the probe for detecting a signal in the range of 5 to 10 MHz emitted by the partial discharge. The detector has an input attenuator circuit which is connected to the probe to lower the level of the detected signal to a desired level. A transformation circuit is further provided to change the detected signal to a predetermined frequency signal substantially free of noise and representative of the magnitude of the detected partial discharge signal. An amplifier circuit amplifies the predetermined frequency signal and an output circuit generates signals indicative of the presence and magnitude of the partial discharge. To determine the location of a partial discharge along a cable or joint, the probe is past gently and slowly over the insulated area and the detector meter is carefully observed to see where the readings reach maximum.
EP 800652 relates to methods and apparatus for locating incipient faults in electric power distribution cables which include the application of an excitation voltage to a power line to produce a partial discharge signal pulse at a fault along the power line. The surface of the power line is scanned with two sensors spaced apart along the power line and disposed adjacent the surface of the power line, to detect the partial discharge signal pulse to produce discrete detected pulses. These detected pulses are combined to produce a combined signal having an amplitude level which reaches an extreme value when the fault is located equidistantly between the sensors. The sensors are moved along the power line until the fault is located substantially equidistantly between the sensors as indicated by the extreme amplitude level of the combined signal.